Comparison with Results Obtained by Others

The experimental study of the effective electron mass ofκ-(ET)₂Cu(NCS)₂ has been mo-tivated mainly by previous results obtained by Meyer, Nguyen and Steep [37][93]. The results presented in the previous section will now be compared with the results of these authors. The results obtained by Wosnitza and co-workers [116] are also considered as these represent the first systematic study of the effective electron mass of the compound in question.

The F_α-mass

The results of this work are compared to the results obtained by Nguyen [37] and those of Wosnitza and co-workers [116].

Figure6.10 shows the results of this work in comparison with Nguyen’s.

Both sets of data show considerable deviations from the 1/cosθbehaviour in the angle range of about 20^◦ to 45^◦. In the range ≤ 20^◦, Nguyen’s results seem to be consistent with a 1/cosθ model, although with a slightly smaller zero angle mass than in the data obtained within this work. Also in the range of deviations from the 1/cosθ-expectation, Nguyen’s masses are slightly below the ones obtained within the framework of this work.

If an overall shift of about 0.15meis assumed, the two sets of data would show roughly the same effective mass values. This latter observation seems to hold for the whole depicted angular range, in the regular and the irregular parts.

There seems to be a minimum around 26^◦ to 27^◦, although no data is available for this range. This corresponds to the angle where a maximum deviation of the de Haas-van Alphen amplitude as function of temperature from the Lifshitz-Kosevich theory was ob-served in this work. Nguyen also obob-served the deviation from Lifshitz-Kosevich behaviour at this angular range as she states in [37].

A remarkable qualitative feature that is visible in the data obtained within the frame-work of this frame-work and that does not appear in Nguyen’s data is the relatively high mass

Figure 6.10: Effective electron mass analysis for the F_α mass by Nguyen compared to the results of this work. A plot of 3.25/cosθis also shown. Nguyen’s results are from [37].

at 30.3^◦. One could suspect some sort of sharp maximum there, but on the other hand, the error becomes excessive there as a result of the non-Lifshitz- Kosevich temperature dependence of the de Haas-van Alphen amplitude there so that this cannot be said with certainty.

Summing up, it can be said that the occurence of non-Lifshitz-Kosevich dependencies of the de Haas-van Alphen amplitude on angle and temperature for the range from 20^◦ to 45^◦ appears in both measurements. The exact manifestation of the irregularity is not the same for the Fα-mass, but not contradictory either if one accounts for the experimental errors.

Finally, the results of Wosnitza et al. [116] are to be considered. They are shown in figure 6.11 and apply for the F_α-frequency. Although the authors concluded at the time that the mass follows a 1/cosθ dependence for the whole examined range, it is worth noting that on their side of negative angles the experimental data is scattered further off the fit function than it is the case on the side of positive angles. This was not considered as significant at the time, but may be of interest in the present context, where an asymmetric deviation is discussed. Wosnitza et al. do not show any data for the range of about 26^◦ to 27^◦. It seems however worth noting that also in their data there is one point considerably above the others, that is the one at about -40^◦. It is, however, about 10^◦ off the angle at which the peak value was observed in this work. Wosnitza et al. obtained a zero angle mass of (3.24±0.2)m_e for the best 1/cosθ fit which is consistent with the result of (3.25±0.2)me obtained in this work. Although some irregularities can also be seen in

Figure 6.11: The Fα mass as function of the angle as obtained by Wosnitza et al.. From [116].

the results of Wosnitza et al. and although these were not regarded as significant by the authors at the time, this is not necessarily a contradiction to the findings of Nguyen and the ones presented in this work.

The F_2α-mass

Nguyen’s results for the F_2α-mass are presented in figure6.12together with the results of this work to be compared.

Comparing the masses obtained in this work to Nguyen’s results as shown in figure 6.12, an agreement within the experimental error can be stated. Around 35^◦, both graphs show masses that are scattered around about 3me (in p= 2 convention). At about 20^◦, Nguyen obtains a mass of 2m_e which overlaps with the value of 2.2m_e found in this work within error bars. Nguyen’s data seem to indicate a minimum around 26^◦, although she was not able to perform an effective electron mass analysis for the 2αorbit at this position.

It seems however worth noting that this suspected minimum corresponds to the position where the strongest deviations of the de Haas-van Alphen amplitude as function of the temperature were observed in this work. The qualitative behaviour of the effective electron mass coming up from lower values at angles around 35^◦can also be seen in Nguyen’s results.

Asymmetry

Nguyen [37] also reports an asymmetry for the two directions of tilting around a specific rotational axis. However, she reports it only for the rotation around the b-axis, that is orientation 1 in the convention of this work. She explains with the asymmetrical bending

Figure 6.12: Effective electron mass analysis for theF2α mass by Nguyen compared to the results of this work. Nguyen’s results have been divided by 2 in order to be comparable to the results of this work considering the different conventions used. Nguyen’s results are from [37].

of the anion molecule within the crystal structure.

Summary of the Comparison with Other Results

Nguyen [37] finds deviations from the 1/cosθ behaviour for both the F_α and the F_2α mass in the same angular ranges. Whereas for the F_2α-mass, the agreement between the two sets of data is quite well within the experimental error, the qualitative shape of the irregularity is difficult to determine, mainly due to the irregular dependency of the de Haas-van Alphen amplitude on the temperature in the angular range concerned.

However, the irregularities could be confirmed in their existence for the same angular range and they do not contradict each other within the experimental error, which is however -large, again as a result of the irregular temperature dependence already mentioned. The results of Wosnitza et al. [116] do also show some asymmetry around the zero position, but the effect is less pronounced so that it was not regarded as significant by the authors at the time. The observed asymmetry is in accordance with statements by Nguyen on the subject and possibly due to the asymmetrical bending of the anion molecule [37].

6.6.3 Usefulness of the Lifshitz-Kosevich Theory for κ-(ET)₂Cu(NCS)₂,